It has been proposed in French Pat. No. 2,362,886 to prepare anionic resins with high density by bromination, chloromethylation and amination of particles of crosslinked styrene copolymers. This patent suggests a dibromination of the crosslinked styrene copolymer, which lead to a decrease in reactivity of the copolymer and tends to prevent the attachment of activatable groups by chloromethylation.
The present invention aims to provide a simpler process for preparing high density anionic resins by bromination. It has been discovered that it is possible to eliminate the expensive and dangerous step of chloromethylation if crosslinked vinyltoluene is used as the copolymer, and if this copolymer is dibrominated to permit not only the attachment of a bromine atom on the ring but also on the lateral chain. Thus there is obtained both a higher density resin and the presence of bromomethyl groups able to attach nitrogenous active groups by amination, without conducting an intermediate haloalkylation.